The invention refers to an apparatus for transferring individual articles in sheet form and placing them in stacks of pre-determined numbers. It includes a conveyor which picks up said articles from a delivery system by gripping them along their edges. Along said conveyor, at least two reciprocating expellers remove said articles from the conveyor means and lay them down onto a stacker. The expellers are disposed in succession in the direction of advancement of said conveyor and caused to work alternately whereby, at selected times, the article fed to the expeller which is furthest away from the delivery system passes the temporarily inoperative expeller which lies closer to the delivery system.
In the production of paper napkins, especially the type double-folded along two folding lines perpendicular to the sides of the manufactured article, machines are well-known in the art wherein a web of paper material is longitudinally slit into strips which are then fed to transverse cutting and folding units. The finished article is withdrawn by a takeout cylinder, on which it has been folded, by a belt system which then conveys it to the expeller or extractor having reciprocating means to remove each individual folded article from the conveyor and lays it down where it becomes a part of a stack of articles. The expeller is programmed so that each stack contains a predetermined number of articles which must be moved away from the stacking region upon completion thereof. In order to operate the production machine continuously, without any slowing down or dwell, the prior art suggests the use of two symmetrical units for the withdrawal and stacking of the articles. The two units work alternately so that when one of them has completed a stack or pack, the subsequent articles are directed towards the other unit, thereby allowing the previously formed stack to be unloaded. Such machines are disclosed, for example, in the U.S. Pat. Nos. 5,088,975 and 4,921,235.
These known apparatuses are provided with two side-by-side rotating cylinders, each of which is associated with a respective belt conveyor with relevant expeller member. A selecting device diverts the articles coming from the production machine towards one or the other conveyor. This requires a considerably larger machine, and the use of a particularly delicate diverting mechanism which is subject to malfunctions.
European patent No. 076,939 shows an apparatus of the above-mentioned type, which has some of the features of this invention. In such apparatus, provision is made for a single conveyor system for the folded articles to which two expellers or extractors are combined, and which operate alternately. When the expeller or extractor member which is closest to the article-delivery region is not operating (i.e., when the folded article is to be conveyed directly to the second expeller), the first expeller is blocked in a withdrawn position relative to the trajectory of the folded article. Such system, therefore, requires means which are able temporarily to stop the reciprocating motion of at least one of the two extractors. In addition to the disadvantage of needing auxiliary means for stopping the motion, which must intervene at a very high speed, this prior device is unable to reach high production rates, because it requires some time for the activation and deactivation of one of the expellers or extractors. The time required for this operation is the elapsed time between the movement of successive articles along the conveyor means.